1. Field of the Invention
This invention relates to a method and apparatus for measuring the weight of a seat occupant. Specifically, a simplified sensor arrangement is mounted within a vehicle seat to measure only vertical forces while canceling out lateral and longitudinal forces to provide an accurate weight measurement.
2. Related Art
Most vehicles include airbags for the driver and passenger. It is important to control the deployment force of the airbags based on the size of the driver or the passenger. One way to control the deployment force is to monitor the weight of the seat occupant. If a smaller person such as a child or infant in a car seat is in the passenger seat, the weight on the seat will be less than if an adult occupies the seat.
Current systems for measuring the weight of a seat occupant are complex and expensive. Sensors are placed at a plurality of locations in the seat bottom and the combined output from the sensors is used to determine the weight of the seat occupant. Each sensor experiences a substantially vertical force, due to the weight of the seat occupant, but is also subject to longitudinal and lateral forces caused by acceleration, deceleration, or turning. The lateral and longitudinal forces picked up by the sensor incorporate an error component into the weight measurement. Current seat weight sensors are very sophisticated using multiple strain gages and complicated bending elements to provide high measurement sensitivity in the vertical direction and low sensitivity to lateral and longitudinal forces.
One weight measurement system corrects for this error by providing a double bending beam in each sensor. Thus, each sensor is made up from two (2) separate half-bridge strain gages that are diametrically opposed to cancel forces in a longitudinal or lateral direction. The two half-bridge strain gages are connected to form a full Wheatstone bridge. By doing this, the differential of the signal generated by each half-bridge strain gage is the output.
An error force in the direction of the strain gage grids appears on each half-breed strain gage equally, and the differential is zero (0). Forces occurring in other directions are generally not significant to the generation of the signal due to being perpendicular to the strain gage grid. Thus, a characteristic of the double bending beam sensor is to cancel the forces along the axis parallel to the gage grids. The forces along the perpendicular axis are not problematic because these forces are perpendicular to the gage grid. This allows error to be eliminated or canceled out at each sensor location in the seat. The output from the combined sensors is then added together to determine the weight of the seat occupant. The use of a double bending beam for each sensor location is very expensive.
Thus, it is desirable to have an improved seat occupant weight measurement system that is simplified and inexpensive yet provides accurate measurements by eliminating error caused by lateral and longitudinal forces.
In a disclosed embodiment of this invention, a system for measuring the weight of a seat occupant includes a seat bottom for receiving a substantially vertical seat occupant weight force and at least one pair of sensors. A first sensor is mounted within a first portion of the seat bottom and a second sensor is mounted within a second portion of the seat bottom. The first sensor generates a first weight signal comprised of a first vertical force component and a positive error component induced by application of non-vertical seat forces to the seat bottom. The second sensor generates a second weight signal comprised of a second vertical force component and a negative error component induced by application the non-vertical seat forces. A processor is used to determine seat occupant weight based on the first and second weight signals. Seat occupant weight is determined by summation of the first and second vertical force components and adding the positive and negative error components eliminates error induced by the non-vertical seat forces.
In a preferred embodiment, the seat bottom is divided into a first quadrant, a second quadrant, a third quadrant, and a fourth quadrant. The first sensor is mounted within the first quadrant for generating the first weight signal comprised of the first vertical force component and a first error component induced by application of non-vertical seat forces to the seat bottom and the second sensor is mounted within the second quadrant for generating the second weight signal comprised of the second vertical force component and a second error component induced by application of non-vertical seat forces. A third sensor is mounted within the third quadrant for generating a third weight signal comprised of a third vertical force component and a third error component induced by application the non-vertical seat forces. A fourth sensor is mounted within the fourth quadrant for generating a fourth weight signal comprised of a fourth vertical force component and a fourth error component induced by application the non-vertical seat forces. The first and third sensors are preferably orientated within the first and third quadrants, respectively, such that the first and third error components are generated as positive errors. The second and fourth sensors are orientated within the second and fourth quadrants, respectively, such that the second and fourth error components are generated as negative errors. The processor determines seat occupant weight based on the first, second, third, and fourth weight signals and adding the positive and negative errors eliminates the error induced by the non-vertical seat forces.
A method for determining the weight of a seat occupant includes the following steps. A vertical occupant force is applied against a seat bottom. A first weight signal is generated that has a first vertical force component and a positive error component induced by application of a non-vertical force to the seat bottom. A second weight signal is generated that has a second vertical force component and a negative error component induced by the non-vertical force. The first and second weight signals are combined and the error induced by the non-vertical force is canceled by adding the positive and negative error components together. The first and second vertical force components are added together to determine seat occupant weight.